The invention relates to a method for the manufacture of a cold drawn wire of a precipitation hardenable stainless steel. The invention also relates to the cold drawn wire and to precipitation hardened springs made of the cold drawn wire. Typically, the stainless steel in the springs consists of so called 17-7 PH steel.
The precipitation stainless steel that contains appr 17% Cr, appr 7% Ni, and any precipitation hardening element, normally Al, was developed during the 1940""ies. It was disclosed in an article in the Iron Age, March 1950, pp 79-83. Already in this article, the suitability of the steel as a material for springs was suggested. Good spring features in combination with a good corrosion resistance have made the steel widely used as a spring material in corrosive environments. An environment of that type is injections pumps for Diesel engines, more particularly turbo Diesel engines. Springs which are used for this purpose must have a good corrosion resistance, which 17-7 PH steels have, in combination with a very high fatigue resistance of the springs. The latter condition, however, has been difficult to achieve. It has been known for long that the fatigue resistance to a high degree depends on the surface of the spring wire. In order that the spring shall have a high fatigue resistance, the wire must not have any visible defects, which can initiate fatigue failures. Nor shall the surface layer contain any large slag inclusions or large zones containing major accumulations of smaller slag inclusions, which also can initiate failures. These conditions, as far as the slag picture is concerned, have been difficult to satisfy and have caused significant rejection of wire that does not meet with the stipulated quality requirements. This in its turn has the effect that the wire material that has been approved in thorough quality control necessarily becomes very expensive. Nevertheless, one can not say that the material satisfies highest demands as far as fatigue resistance is concerned.
It is a purpose of the invention to provide a solution of the above mentioned problems. The invention herein is based on the observation that large slag inclusions and zones of the above mentioned type in the surface layer of the rolled wire can be avoided or significantly reduced if the steel is electro slag refined, i.e. subjected to the treatment which is known under the short name ESR (=Electro Slag Refining, also referred to as Electro Slag Remelting). At the ESR treatment there can be used a conventional slag mixture which is used according to known technique, and which at the ESR remelting process forms a melt, in which the electrode that shall be remelted is molten off drop-wise, such that the drops will sink through the slag melt to an underlying pond of molten metal, which solidifies successively to form a new ingot. For example, a slag mixture can be used, which is known per se, and which contains appr 30% of each of CaF2, CaO, and Al2O3 and normally a certain amount of MgO in lime fraction as well as one or a few percent SiO2. In the case when the melting electrode, as according to the invention, consists of a stainless 17-7 PH steel, which contains slag inclusions of varying sizes, the remelted ingot will get a different slag picture than before the remelting operation. It appears that the ESR slag functions as a screen for larger slag particles existing in the steel prior to the remelting operation. At least this appears to be true for those slags which have proved to have a detrimental effect on the fatigue strength of the spring wire, namely slags of type CaO, Al2O3, and MgO. While the smaller slag inclusions become more evenly distributed and possible zones of slag accumulations become smaller and therefore more harmless, the amount of smaller slag inclusions of this type in the remelted material is influenced only to a low degree. The fatigue tests which have been performed with conventional materials and with materials according to the invention show that the critical slag size limit lies between 20 and 30 xcexcm. Therefore, slag inclusions larger than 30 xcexcm shall be avoided. Preferably, the wires should not contain slag particles larger than 25 xcexcm.
The steel that is used according to the invention may have a chemical composition which is well known in the art and which as a matter of fact is standardized since long
The method of the invention for the manufacture of a cold drawn wire of a precipitation hardenable stainless steel comprises the following steps:
preparation of a melt, which besides iron contains in weight-%
0.065-0.11% C.
from traces to max 1.2% Si
0.2-1.3 Mn
15.8-18.2% Cr
6.0-7.9% Ni
0.5-1.5% Al
totally max 2% of other, possibly existing alloying elements;
casting the prepared melt to form ingots or, preferably, a strand, which is cut up into sections;
electro slag refining said ingot or cut-up strand, possibly after forging and/or rolling to the shape of electrodes suitable for electro slag refining, to form ESR ingots;
hot working said ESR ingots, said hot working being finished by wire rolling, followed by pickling for the formation of a pickled, rolled wire,which in a surface layer thereof, to the depth of 1 mm counted from the surface, in a longitudinal, central section through the wire, does not contain slag inclusions larger than 30 xcexcm, preferably max 25 xcexcm; and
cold drawing the wire with at least 30% reduction.
Al is added as a subsequent operation, when the molten metal has got its intended basic composition through conventional steel manufacturing practice, suitably in a ladle treatment process which follows subsequent to decarburisation in a converter.
During the ESR remelting operation, a certain amount of that aluminium, which was added in connection with the initial preparation of the molten metal, can be lost. Therefore, in connection with the ESR remelting operation, more aluminium ought to be supplied to the melting pond for the replacement of any losses, so that the ESR ingot obtained after the ESR remelting operation will contain 0.5-1.5 Al.
More specifically the invention relates to the manufacture of a precipitation hardenable stainless steel according to the method that is described in the foregoing, which steel besides iron contains in weight-%:
0.3-0.1, preferably max. 0.09 C.
0.1-0.8, preferably 0.2-07 Si
0.5-1.1, preferably 0.7-1.0 Mn
max. 0.05, preferably max 0.03 P
max 0.04, preferably max 0.02 S
16.0-17.4, preferably 16.5-17.0 Cr
6.8-7.8, preferably 7.0-7.75 Ni
0.6-1.3, preferably 0.75-1.0 Al
max 0.5 Mo
max 0.5 Co
max 0.5 Cu
max 0.1, preferably max 0.05 N
max 0.2, preferably max 0.01 Ti
Helicoidal springs are spun in a conventional mode of the cold drawn wire according to the invention. The springs are precipitation hardened through heat treatment at a temperature of 450-500xc2x0 C. for 0.5-2 h, suitably at appr 480xc2x0 C. for 1 h. followed by cooling in air. The structure of the material in the finished springs consists of 50-70 volume-% tempered martensite containing precipitated phases of aluminium and nickel in the martensite, preferably AlNi3, remainder austenite and max 5% xcex4-ferrite.